Bentonite is supplied on the market in a powdered form having a particle size of not greater than 150 .mu.m, which is prepared by coarsely crushing ore from a bentonite mine, which contains about 10 to 40 wt % of water, to a size of about 10 mm or less, drying it in a rotary drier to reduce the water content to about 6 to 10 wt %, and pulverizing it in a roller mill or a hammer mill to the above size.
A soil stabilizing fluid used in civil engineering works, such as underground continuous wall construction or in-situ piling, and a drilling fluid (mud) used in development of underground resource energy, for example, drilling of an oil well, a geothermal well or a spring well, is generally prepared by dispersing powdered bentonite in fresh water in a concentration of 4 to 8 wt % by stirring it in a stirring machine and, if desired, adding adequate amounts of chemical substances, such as sodium carboxymethyl cellulose, polymers and surface active agents, in order to prevent deterioration due to contamination with cement or a brine such as seawater. On being poured into water, powdered bentonite first forms lumps, which are then disintegrated by stirring it a long time into a uniform dispersion. In what follows, the single term "lumps" means undissolved lumps of powder which are formed when the powder is poured into water. Therefore, preparation of a soil stabilizing slurry or mud has required great energy and time for stirring. The higher the viscosity and water impermeability of bentonite due to the higher bonding force with water, that is, the higher the quality of bentonite, the greater the tendency to formation of lumps of powder which are hard to disperse in water.
As the depth reached by recent ground drilling works has increased, the demand for a stabilizing slurry or mud having high water impermeability sufficient to withstand high underground water pressure has been increasing. While bentonite of high quality meeting that demand has been supplied, bentonite satisfying the requirements for easy dispersion as well as high quality has been keenly sought.
Efforts have hitherto been made to satisfy the above requirements. For example, a grinding method for minimizing formation of fine particles of 75 .mu.m or smaller which are apt to form lumps was suggested. Removal of fine particles by air classification was also attempted. However, all the conventional attempts failed to achieve satisfactory results.
Bentonite is an alkaline clay mainly comprising a clay mineral montmorillonite and, unlike other clay minerals such as kaolinite or pyrophyllite, has a great cation-exchange capacity of 40 to 120 milliequivalents per 100 g of clay. The exchangeable cations include a sodium ion, a calcium ion, and a magnesium ion. In particular, bentonite having an abundance of sodium ions has such high hydrophilic properties that it absorbs a large quantity of water to increase its own volume remarkably, that is, it exhibits high swellability in water and is finally dispersed to colloidal particles and therefore has been used as a fundamental element of a stabilizing slurry or mud for drilling or boring works. Essential functions required for the material of stabilizing slurries or mud are proper viscosity and water impermeability.